A fiber Bragg grating (FBG) is a permanent periodic refractive index modulation in the core of a single-mode optical silica glass fiber over a length of typically 1-100 mm. It can be created in a photosensitive fiber (i.e. a fiber including a photosensitive dopant, such as germanium) by transversely illuminating the fiber with a periodic interference pattern generated by ultra-violet (UV) laser light. The refractive index modulation in a standard FBG is believed to be formed by UV-induced breaking of electronic bonds in the Ge-based defects, releasing electrons which are thereafter re-trapped at other sites in the glass matrix. This rearrangement of the bonds causes a change in the fiber's absorption spectrum and density, thereby changing the refractive index of the glass. It is well-known that an FBG reflects light within a narrow bandwidth (typically 0.1-0.3 nm), centered at the Bragg wavelength λB=neffΛ, where neff is the effective refractive index seen by the light propagating in the fiber, and A is the physical period of the refractive index modulation.
It is known that the reflected Bragg wavelength λB from an FBG will change with any external perturbation which changes the effective refractive index seen by the propagating the light and/or the physical grating period (Λ), such as temperature and strain. By measuring the reflected Bragg wavelength λB (using, for example, a broadband light source and a spectrometer), an FBG can be used as a sensor for measuring such external perturbations. A standard UV-induced FBG can be made thermally stable up to 150-200° C. and thus used as a sensor up to this limit. Unfortunately, at higher temperatures the UV-induced refractive index modulation decays and the grating is erased.
FBGs can also be used as a pressure sensor by measuring the shift in Bragg wavelength caused by hydrostatic pressure-induced compression of the silica glass fiber. An FBG pressure sensor can be made with relatively small dimensions, good reproducibility and long-term stability, provided by the all-silica construction of the sensor. An all-fiber FBG sensor with enhanced pressure sensitivity and inherent temperature compensation can be made by using a passive or an active fiber laser FBG written in a birefringent side-hole fiber, the fiber having two open channels (holes) symmetrically positioned on each side of the fiber core. See, for example, U.S. Pat. Nos. 5,828,059 and 5,841,131. It is also possible to make FBG pressure sensors with enhanced pressure sensitivity by using a glass transducer element surrounding the optical fiber, either to convert pressure to strain/compression in the fiber or to convert pressure to fiber birefringence.
In addition, diffusion of gases (such as hydrogen) into the core of the fiber will cause a change in the refractive index proportional to the hydrogen concentration, and consequently modify the Bragg wavelength of an FBG written into the core of the fiber. Hydrogen will also cause an increase in signal loss along an optical fiber, which has been found to be detrimental for FBG-based rare-earth doped fiber lasers. Finally, diffusion of gases into the holes of a side-hole fiber will change the pressure inside the holes, and hence the pressure difference which affects the measurement of the external hydrostatic pressure.
U.S. Pat. No. 5,925,879 discloses the use of a carbon coating on an FBG sensor to protect the optical fiber and sensors when exposed to a harsh environment. Carbon has been shown to provide a good hermetic coating for optical fibers, making them essentially impermeable to both water and hydrogen, thus maintaining the mechanical strength and low loss of the fiber. A carbon coating can be applied to an optical fiber during the drawing process before the fiber glass cools through a pyrolytic process (see, for example, U.S. Pat. No. 5,000,541). Carbon coating using a similar technique can also be applied to splices between hermetic fibers to maintain hermeticity after splicing of carbon-coated fibers, as disclosed in U.S. Pat. No. 4,727,237. In the latter patent, a pyrolytic technique is used based on heating the fiber splice region with a CO2 laser inside a chamber containing a reactant gas, causing a carbon coating to form on the glass surface by pyrolysis of the reactant gas. However, the temperature in the fiber needs to exceed 1000° C. to provide highly hermetic coatings. A standard FBG in a germanium-doped silica fiber cannot be carbon coated using such a process since the grating will be erased, as discussed above, by the high temperature involved in the process.
Thus, a need remains in the art for a technique to form protected FBGs that may be used in high temperature, harsh environments without experiencing the hydrogen-induced losses associated with the prior art.